Gas liquid chromatography-Mass Spectrometry (GLC-MS), ion exchange chromatography, high pressure liquid chromatography (HPLC) and other separation procedures are used to isolate and measure the specific activities of deuterium-labelled or 14C-norepinephrine (NE) metabolites, 3-methoxy-4-hydroxyphenylglycol (MHPG) and vanillyl mandelic acid (VMA) in the plasma and urine. D,lMHPG-D3 and D(-)MHPG-D3 have been injected into human controls and the kinetics of conversion to VMA-D3 studied in blood and urine. Similar studies are planned in patients with orthostatic hypotension as well as with various neurologic and mental disorders.